The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A hybrid vehicle includes an internal combustion engine and an electric motor to provide vehicle propulsion. Hybrid vehicles include series hybrids and parallel hybrids. In series hybrids, an engine is provided to run a generator that produces power for vehicle propulsion. In parallel hybrids, the electric motor and engine may propel the vehicle individually, or together; with the engine additionally charging the power source (batteries) for the motor.
Hybrid vehicles tend to improve fuel economy and reduce vehicle emissions by using only the electric motor while the vehicle is idling or moving at low speeds. Hybrid vehicles may carry a smaller engine than typical vehicles and may be configured to run the engine when the electric motor's rechargeable battery charge falls below a predetermined level or when the hybrid vehicle needs the additional horsepower. Typical conditions requiring the additional horsepower may include rapid acceleration and high load demands on the electric motor and vehicle. Such operation of the vehicle may result in the engine frequently starting and stopping.
To reduce emissions, engines control the amount of fuel that is burned. Engine control systems control an air-fuel ratio with a goal of reaching an optimum stoichiometric ratio. At optimum stoichiometric ratio, all of the fuel is burned using all of the oxygen in the air.
Most modern vehicles are equipped with three-way catalytic converters. “Three-way” refers to the three emissions that catalytic converters help to reduce—carbon monoxide, volatile organic compounds (VOCs) and NOx. The catalytic converter uses two different types of catalysts, a reduction catalyst and an oxidation catalyst. Both types include a ceramic structure that is coated with a metal catalyst, usually platinum, rhodium and/or palladium. The catalytic converter exposes the catalyst to the exhaust stream while minimizing the amount of catalyst that is required due to the high cost of the catalyst materials.
There are two main types of structures that are used in catalytic converters—honeycomb and ceramic beads. The reduction catalyst is the first stage of the catalytic converter that typically uses platinum and rhodium to help reduce the NOx emissions. When the NOx molecules contact the catalyst, the catalyst separates the nitrogen from the molecule, holds on to the nitrogen and frees the oxygen in the form of Ox. The nitrogen bond with other nitrogen that are also held by the catalyst, forming N2. For example:2NO=>N2+O2 or 2NO2=>N2+2O2 
The oxidation catalyst is the second stage of the catalytic converter that reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. The oxidation catalyst reacts the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example:2CO+O2=>2CO2 
The third stage is a control system that monitors the exhaust stream and uses the information to control the fuel injection system. Typically an oxygen sensor is mounted between the engine and the catalytic converter. The oxygen sensor senses oxygen in the exhaust. An engine control system increases or decreases the amount of oxygen in the exhaust by adjusting the air-fuel ratio. The engine control system makes sure that the engine is running at close to the optimum stoichiometric ratio and that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and CO.
The catalytic converter only works at a fairly high temperature. When the engine is first started, the catalytic converter is not effective in removing emissions in the exhaust until the catalytic converter reaches an operating temperature called the light-off temperature. “Light-off temperature” is the point where the conversion of CO or HC has reached 50% efficiency. Starting an engine with a catalytic converter that needs to be warmed up to the light-off temperature, or cold-starting, may be a repetitive act particularly seen in hybrid vehicles that repeatedly start, stop and restart the engine during normal operation.
One conventional solution to engine cold-starting is to move the catalytic converter closer to the engine. Hotter exhaust gas reaches the catalytic converter and heats it up faster. This approach tends to reduce the life of the catalytic converter by exposing it to extremely high temperatures. Typically, the catalytic converter is positioned under the front passenger seat; far enough from the engine to keep the temperature down to levels that will not harm it.
Preheating or supplementally heating the catalytic converter is another conventional way to reduce the time required for the catalytic converter to reach the light-off temperature. The easiest way to heat the converter is to use electric resistance heaters, such as found in heating elements. These “external” heaters are placed upstream from the catalytic converter, supplementally heating the passing exhaust gases that enter the converter. The heating element may contain catalytic material. Once the catalyst associated with the heating element reaches light-off temperature, engine exhaust gas will oxidize while passing over the heating element catalyst. This oxidation releases additional heat into the exhaust gas, rapidly elevating the catalytic converter temperature to light-off as well.
The 12-volt electrical systems on most vehicles will not provide enough energy to pre-heat the heating element fast enough. The driver may have to wait several minutes for the heating element to be pre-heated before starting the vehicle. Without preheating, exhaust gas flowing past the heating element will cool the catalyst associated with the heating element and increase the amount of time needed to reach to light-off temperature.